1. Field of the Invention
The present invention relates to a color filter for a liquid crystal display or the like. More particularly, the present invention relates to a mask for sputtering to be used for forming a transparent electrode layer on a color filter by sputtering, a sputtering process, a color filter provided with a transparent electrode layer formed by the sputtering process using the mask for sputtering.
2. Description of the Related Art
A color filter fabricating method includes a chromium film forming process for forming a chromium film on a glass substrate by a vacuum evaporation method; a black shading layer forming process for forming a black shading layer of a striped pattern having stripes or a latticed pattern having crisscrossed stripes by coating the chromium film with a photoresist film, placing a photomask on the photoresist film, exposing the photoresist film through the photomask to light and developing the exposed photoresist film, etching the chromium film through the exposed photoresist film, and removing the exposed photoresist film; RGB color layer forming process for forming R, G and B color layers; and a transparent electrode forming process for forming a transparent electrode layer by forming an indium tin oxide film (ITO film) on the R. G and B color layers. The RGB color layer forming process includes the step of forming a first color layer by spreading a first coloring photosensitive material in a first color film over the black shading layer, placing a photomask on the first color film, exposing the first color film to light through the photomask and developing the exposed first color film, and steps of forming a second and a third color layer, similar to the step of forming the first color layer.
A sputtering method is used prevalently in the transparent electrode forming process because the sputtering method is capable of forming a sufficiently adhesive thin film of a uniform thickness at a comparatively low temperature in a short time. In an automated production line, a combination of a glass substrate provided with a black shading layer and the color layers, and a sputtering mask is fixedly held in a jig, and several tens of such jigs are carried into a sputtering apparatus, and the glass substrates held in the jigs are subjected to a sputtering process.
FIG. 2 shows a sputtering jig disclosed in JP-A No. 117839/1993 in an exploded perspective view. The sputtering jig 1 has a substrate holding plate 2 provided with an opening 2a, and a substrate fastening plate 5 connected to the substrate holding plate 2 by a hinge 3 so as to be turnable relative to the substrate holding plate 2. A plurality of magnets 6 are embedded in the substrate fastening plate 5. A plurality of guide pins 7 are arranged on the substrate holding plate 2 around the opening 2a. A mask 8 and a glass substrate 9 are set in place on the substrate holding plate 2 by setting the mask 8 and the glass substrate 9 in contact with the guide pins 7. After setting the mask 8 and the glass substrate 9 on the substrate holding plate 2, the substrate fastening plate 5 is turned relative to the substrate holding plate 2 to hold fixedly the mask 8 and the glass substrate 9 between the substrate holding plate 2 and the Lifer substrate fastening plate 5. The glass substrate 9 is provided with a plurality of color pattern films 10, and the mask 8 is provided with openings 8a of a shape corresponding to that of the color patterns 10 on the glass substrate 9. The mask 8 is formed of a heat-resistant ferromagnetic material capable of withstanding a high temperature on the order of 200.degree. C.
In the jig 1, the mask 8 of a ferromagnetic material is attracted firmly to the substrate fastening plate 5 by the magnets 6 of the substrate fastening plate 5 so that the mask 8 is in close contact with the glass substrate 9. When the mask 8 and the glass substrate 9 thus firmly joined together are exposed to a high temperature on the order of 200.degree. C. in the sputtering apparatus, thermal stresses are induced in the mask 8 and the glass substrate 9 due to difference in coefficient of thermal expansion between the mask 8 and the glass substrate 9. Consequently, peripheral portions (black shading strips 10b) of the color pattern films 10 formed on the glass substrate 9 are liable to be damaged, and liquid crystal displays provided with color filters thus fabricated are liable to be defective due to the leakage of light thrown by a backlight.
A sputtering jig is proposed to solve such a problem in JP-A No. 88206/1994. When this prior art sputtering jig is used, recesses are formed in brim portions of the openings 8a of the mask 8 (FIG. 2) facing the glass substrate 9 by etching so that clearances are secured between the mask 8 and the circumferences of the color pattern films 10 formed on the glass substrate 9 to prevent damaging peripheral portions of the color pattern films 10.
A sputtering apparatus that processes the glass substrate 9 held by the sputtering jig has a vacuum vessel filled with a gas. A negative voltage is applied to a target placed in the vacuum vessel to produce a plasma by ionizing the gas by electric discharge, and a transparent electrode layer is deposited on the color pattern films 10 by bombarding the target with ions of the plasma to cause the target sputter atoms from its bombarded surface. When a gap 11 is formed between the mask 8 and the glass substrate 9 as shown in FIG. 3, a relatively amount of electric charges is accumulated on the surface of the mask 8 due to secondary electron emission because the mask 8 and the glass substrate 9 are exposed to electric discharge. Therefore, abnormal electric discharge from the mask 8 to the glass substrate 9 occurs when the potential difference between the mask 8 and the glass substrate 9 increases to a certain level. Consequently, breaks F are formed in peripheral portions (black shading strips 10b) of the color pattern films 10 formed on the glass substrate 9 due to dielectric breakdown.